This invention relates to cladding mode pumped amplifiers and to an optical waveguide for use in such amplifiers. It is known to provide optical waveguides such as optical fibres in which a core is suitably doped, for example using rare earth ions, such that the core constitutes a light amplifying medium capable of providing light amplification by stimulated emission of radiation. Such fibres may therefore be used to provide light amplification by transmission through the fibre or used in conjunction with a resonant cavity to provide a laser. The present invention is concerned primarily with light amplification as in the case of signals transmitted down optical fibres.
In order to effect optical pumping in the light amplifying medium, it is necessary to provide pumping light at an appropriate wavelength to excite the amplifying medium. Although pumping light in a conventional amplifier is injected longitudinally into the core together with the optical signal there are practical limitations on the amount of power which can be launched into a single mode core. It has therefore been proposed to inject pumping light to propagate longitudinally in the cladding surrounding the core, such propagation requiring the fibre to be a double clad fibre having an inner cladding surrounding the core and an outer cladding surrounding the inner cladding such that the inner cladding acts as a wave guide providing multi-mode conduction of the pumping light. In such an arrangement, optical pumping occurs due to the overlap of the electric field intensity of pumping light with the core.
A problem with such cladding mode optical pumping is that some of the modes in the pumping light have only minimal overlap with the core and therefore provide virtually no contribution to optical pumping. It has been proposed in U.S. Pat. No. 4,815,079 to improve the efficiency of cladding mode optical pumping by placing the core at an off-centre position within the inner cladding and to further improve pumping efficiency by perturbing the modes in the inner cladding by introducing slight bends in the fibre. The perturbation introduced by the bends causes radiation from cladding modes which would not ordinarily pass through the core to couple into other cladding modes which pass through the position occupied by the core.
A disadvantage of bending the fibre in this manner is that the inter-modal coupling is indiscriminate and uncontrolled. In some applications, it may not be practicable to allow the fibre to be bent. The off-set core configuration may also present difficulties when coupling the conventional fibres having coaxial symmetry.
It is also known from U.S. Pat. No. 5,473,622 to provide a cladding pumped laser in which the inner cladding has a rectangular cross section and a resonance cavity is formed in the core by writing one or more spaced pairs of index gratings, each pair defining an oscillator. The index gratings are written in the core by doping the core with refractive index modifying dopants and transversely illuminating the fibre with a focused ultra-violet beam creating an interference pattern at the core. Index changes occur in the core with a predetermined spatial periodicity where the UV light bands appear, thereby forming a Bragg reflector at the wavelength of the optical signal conducted through the core. Index gratings formed in the core of optical fibres are also known for other purposes such as wavelength selective couplers and taps and dispersion compensated filters.
It is also known from K. O. Hill et al,.Electronics Letters, Aug. 2, 1990, Vol. 26, No. 16, to write long period gratings in the core of a multi-mode fibre with a single cladding in order to couple energy from one mode to another mode during longitudinal transmission through the core. A step by step index grating writing method is disclosed in which individual index perturbations are created with any desired spatial frequency.